how do i find and delete duplicate photos and albums on iPhoto 11?

I have a variety of iPhones and cameras, and import all my photos into iPhone '11 version 9.2.1 (628). For some reason iPhoto imports photos which have already been imported, and when syncing with my iPhones or other computers, replicates the entire library. Resultantly, though I only have around 5,000 photos, my library contains around 25,000 photos. How can I find and delete the duplicate photos and albums? Thanks in advance for your help and happy holidays to all!